Re: Using $# defines a variable?

Short answer: autovivification.

When you write $#{$array}, you're asking Perl what the last index of the array is which the reference $array points to, in other words, you're using the previously undefined $array like an array reference, so Perl autovivifies an anonymous array for you. See also item 3 under Using References.

use warnings;
use strict;
use Data::Dumper;

my $array;
print Dumper($array);
print Dumper($#{$array});
print Dumper($array);

__END__

$VAR1 = undef;
$VAR1 = -1;
$VAR1 = [];
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Re^2: Using $# defines a variable? by LanX (Saint) on May 18, 2019 at 22:14 UTC
Additionally it's worth noting that `$#name` is an lvalue allowing to resize the array `@name` `DB<2> @a=1..10 DB<3> $#a = 3 DB<4> x @a 0 1 1 2 2 3 3 4` [download] In other words autovivification is necessary to allow this feature. Lvalues can be passed around, Perl can't be sure if it's only used for a read access. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l] [select]

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Re^2: Using $# defines a variable?
by LanX (Saint) on May 18, 2019 at 22:14 UTC

$#name

@name

  DB<2> @a=1..10

  DB<3> $#a = 3

  DB<4> x @a
0  1
1  2
2  3
3  4
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In other words autovivification is necessary to allow this feature.

Lvalues can be passed around, Perl can't be sure if it's only used for a read access.

Cheers Rolf
_{(addicted to the Perl Programming Language :)

Wikisyntax for the Monastery
FootballPerl is like chess, only without the dice}

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